The invention concerns a device for drying a material web, specifically paper or cardboard web, of the general type in which a suction guide roll that features a suction device is arranged between two drying cylinders in a row of heatable drying cylinders contacted by the material web. A porous backing belt, preferably a drying wire, runs with the material web through the drying device. The backing belt forces the material web on the drying cylinders and makes direct contact with the suction guide rolls, wherein two adjacent drying cylinders, the suction guide roll arranged in between, and the backing belt define a so-called "wire space". An air guide box is provided on the path of the material web from one of the drying cylinders to the next drying cylinder for feeding drying air directly at the material web by means of a fan featuring a suction line.
Regarding the prior art, reference is made to the Voith patent applications P 38 07 856.2 and P 38 07 857.0. The former application concerns a so-called single-wire drying group. In it, the material web to be dried runs together with a backing belt (preferably drying wire) alternately across heatable drying cylinders and suction guide rolls. In the process, the material web makes direct contact with the drying cylinders, while the backing belt makes direct contact with the suction guide rolls. On its entire path through the drying device the material web is guided by the backing belt, so that the material web must at no point, or at the most at the exit from the drying device, run freely from one roll to another.
The object of the second patent application P 38 07 857.0 is a two-wire drying group. In this group, the material web to be dried meanders alternately across drying cylinders of an upper cylinder row and across drying cylinders of a lower cylinder row. The material web is forced on the upper drying cylinder by an upper backing belt and on the lower drying cylinders by a lower backing belt. Along part of its path between two drying cylinders, the paper web is free, that is, it is not supported by a backing belt.
Common to both configurations is that the material web and the backing belt run jointly from a first drying cylinder to a guide roll looped by the backing belt, whereafter the backing belt runs from this guide roll to another drying cylinder (with the object of the application P 38 07 856.2 the guide roll is fashioned as a suction guide roll). Common to both, additionally, is that the two previously mentioned drying cylinders and the guide roll arranged in between as well as the backing belt define a so-called "wire space." Lastly, common to both is also that for purposes of increasing the specific drying performance at the point where the material web runs from one drying cylinder to the next drying cylinder there is drying air blown directly at the material web. This air absorbs at that point water vapor from the material web. A maximum quantity of this air is directly sucked off as exhaust air. To influence the moisture cross profile of the material web, the supplied drying air flow can, across the width of the material web, be subdivided into individually controllable partial flows.
Although the described measures already permit the expectation of an increased drying performance, additional improvements are desirable. So far, attempts have been made at achieving further improvements, among others, by increasing the exhaust temperature and thus raising the dew point of the exhaust air, additionally by increasing the volumes of supply and exhaust air. However, these measures increase the energy consumption for drying. Besides, there is a risk of creating in the drying device uncontrolled flows of moist exhaust air and that the water vapor will condense on the side or upper walls of the hood, requiring a heavier thermal insulation on the hood walls.
Therefore, the problem underlying the invention is to improve the previously proposed device to the effect that uncontrolled flows of moist air within the drying device will be avoided with greater certainty than heretofore. At the same time, the energy consumption for the drying is to be kept maximally low.